The present invention relates to rubber mixtures containing rubbers polymerised in solution with a secondary hydroxyl group content of 0.1 to 5 wt. %, which groups arc no further than 4 carbon atoms away from the respective side chain end, and to their mixtures with fillers, optionally further rubbers and rubber auxiliary agents and vulcanisates produced therefrom. The rubber mixtures according to the invention are suitable for producing highly reinforced abrasion-resistant moulded articles, in particular for producing tyres which have a particularly high skid resistance in the wet and low rolling resistance.
Double-bond containing anionically polymerised solution rubbers, such as solution polybutadiene and solution styrene6butadiene rubbers have advantages over corresponding emulsion rubbers in the production of low rolling resistance tyre treads. The advantages are inter alia the controllability of the vinyl content and the glass transition temperature associated therewith and the molecular branching. These result in particular advantages in the relationship between skid resistance in the wet and rolling resistance of the tyre in practical application. Thus, U.S. Pat. No. 5 227 425 describes the production of tyre treads from a solution SBR rubber and silica. To further improve the properties, numerous methods for end group modification have been developed, as described for example in EP-A 334 042, with dimethylaminopropyl acrylamide or, as described in EP-A 447 066, with silylethers. Owing to the high molecular weight of the rubbers the content by weight of the end group is low, however, and can therefore influence the interaction between filler and rubber molecule to only a small degree. It was an object of the present invention to produce solution SBR rubbers with a much higher content of active groups for filler interaction.
A further object of the present invention was the production of rubber vulcanisates with a more favourable skid resistance in the wet/rolling resistance relationship. Hydroxyl group-containing solution polybutadiene rubbers are also described in DE-OS 2 653 144. However, these rubbers are not suitable as main components in tyre treads owing to their insufficient strength.
EP-A 464 478 describes a process for hydroxylation of rubbers, the hydroxylated rubbers differing structurally from those of the present invention. In particular, the secondary hydroxyl groups of EP-A 464 478 are located in a less favourable position in the polymer and are therefore far less effective than the hydroxyl groups of the present invention which are located no further than four carbon atoms away from the respective side chain end.
EP-806 452 Al also describes hydroxyl group-containing emulsion and solution rubbers, the hydroxyl proportions for solution rubbers described here lying in a much lower range (0.009 to 0.061%) as a result of the process. The present patent application shows that these proportions do not have any significant influence on the skid resistance in the wet and that the position of the hydroxyl groups is of particular importance.
The earlier applications DE-198 32 459.6 and DE-198 32 458.8 describe hydroxyl group-containing solution rubbers. They do not, however, describe secondary hydroxyl groups and do not mention that the hydroxyl groups are located no more than 4 carbon atoms from the respective side chain end either.
It has now been found that rubber mixtures and rubber vulcanisates with surprisingly improved dynamic cushioning properties in the temperature range (0 to 20 xc2x0 C.) relevant to skid resistance in the wet and in the temperature range (60 to 80 xc2x0 C.) relevant to rolling resistance and improved abrasion behaviour may be produced from hydroxyl group-containing solution vinyl aromatic/diolefin rubbers with a content of 0.1 to 5 wt. % bound secondary hydroxyl groups located no further than 4 carbon atoms from the respective side chain end and with a 1,2 vinyl content of 5 to 60 wt. %. Further surprising advantages were obtained when the rubber mixture was produced not in the kneader as is conventional, but by mixing a solution of hydroxyl group containing rubber and oxidic or silicate-containing filler in organic solvent and subsequently removing the solvent with water vapour as the filler is then precipitated completely with the rubber and does not remain in the waste water, as is the case when using unmodified rubber.